In recent years, the desire to reduce environmental impact has resulted in growing demands for clean liquid fuels that contain minimal amounts of sulfur and aromatic hydrocarbons and are gentle on the environment. As a result of these demands, processes that employ a Fischer-Tropsch synthesis reaction (hereafter abbreviated as “FT synthesis reaction”), which uses a gas containing carbon monoxide gas and hydrogen gas as a feedstock, have begun to be investigated as potential processes that are capable of producing fuel oil base stocks that contain minimal sulfur and aromatic hydrocarbons and are rich in aliphatic hydrocarbons, and particularly kerosene and gas oil base stocks (for example, see Patent Document 1).
The liquid hydrocarbons (raw oil) obtained by the FT synthesis reaction is a mixture containing mainly aliphatic hydrocarbons having a very broad carbon number distribution. From this liquid hydrocarbons can be obtained a naphtha fraction containing a large amount of components having a boiling point lower than approximately 150° C., a middle distillate containing a large amount of components having a boiling point within a range from approximately 150° C. to approximately 360° C., and a wax fraction (hereafter also referred to as the “FT wax fraction”) containing those hydrocarbon components heavier than the middle distillate (namely, components having a boiling point that exceeds approximately 360° C.). Of these fractions, the middle distillate is the most useful fraction, being equivalent to a kerosene and gas oil base stock, and it is desirable to achieve a high yield of this middle distillate. Accordingly, in an upgrading step used for obtaining fuel oil base stocks from the liquid hydrocarbons, the FT wax fraction, which is produced in a reasonably large amount together with the middle distillate during the FT synthesis reaction step, is subjected to hydrocracking to reduce the molecular weight and convert the wax fraction components to components equivalent to the middle distillate, thereby increasing the overall yield of the middle distillate.
Specifically, the FT wax fraction that is obtained from the liquid hydrocarbons by fractional distillation is subjected to hydrocracking in a wax fraction hydrocracking step, and subsequently undergoes gas-liquid separation in a gas-liquid separation step. The thus obtained liquid component is fed into a later stage fractionator together with the middle distillate, which has previously been fractionally distilled from the liquid hydrocarbons and subjected to a separate hydrotreating, and the combined fractions are then subjected to fractional distillation to obtain a kerosene and gas oil base stock.